Why is converting WMV to MP4 so slow

.mp4ffmpegvideo conversionwmv

I am trying to convert a video from WMV to MP4 with FFmpeg but it takes couple of hours. If I try to convert it to AVI it only takes about 10-15 minutes.

ffmpeg version

ffmpeg version N-43206-gf857465
built on Aug  4 2012 16:10:39 with gcc 4.7.1 (GCC)

Conversion to MP4

ffmpeg -i input.wmv -vcodec libx264 output.mp4

libavutil      51. 66.100 / 51. 66.100
  libavcodec     54. 49.100 / 54. 49.100
  libavformat    54. 22.100 / 54. 22.100
  libavdevice    54.  2.100 / 54.  2.100
  libavfilter     3.  5.102 /  3.  5.102
  libswscale      2.  1.100 /  2.  1.100
  libswresample   0. 15.100 /  0. 15.100
  libpostproc    52.  0.100 / 52.  0.100
Input #0, asf, from 'input.wmv':
  Metadata:
    WMFSDKVersion   : 12.0.7601.17514
    WMFSDKNeeded    : 0.0.0.0000
    IsVBR           : 0
    encoder         : Lavf54.22.100
  Duration: 01:14:23.06, start: 0.000000, bitrate: 324 kb/s
    Stream #0:0: Video: msmpeg4 (MP43 / 0x3334504D), yuv420p, 1280x720, 15 tbr,
1k tbn, 1k tbc
[libx264 @ 03427620] using cpu capabilities: MMX2 SSE2Fast SSSE3 FastShuffle Cac
he64
[libx264 @ 03427620] profile High, level 3.1
[libx264 @ 03427620] 264 - core 125 r2208 d9d2288 - H.264/MPEG-4 AVC codec - Cop
yleft 2003-2012 - http://www.videolan.org/x264.html - options: cabac=1 ref=3 deb
lock=1:0:0 analyse=0x3:0x113 me=hex subme=7 psy=1 psy_rd=1.00:0.00 mixed_ref=1 m
e_range=16 chroma_me=1 trellis=1 8x8dct=1 cqm=0 deadzone=21,11 fast_pskip=1 chro
ma_qp_offset=-2 threads=3 lookahead_threads=1 sliced_threads=0 nr=0 decimate=1 i
nterlaced=0 bluray_compat=0 constrained_intra=0 bframes=3 b_pyramid=2 b_adapt=1
b_bias=0 direct=1 weightb=1 open_gop=0 weightp=2 keyint=250 keyint_min=15 scenec
ut=40 intra_refresh=0 rc_lookahead=40 rc=crf mbtree=1 crf=23.0 qcomp=0.60 qpmin=
0 qpmax=69 qpstep=4 ip_ratio=1.40 aq=1:1.00
Output #0, mp4, to 'output.mp4':
  Metadata:
    WMFSDKVersion   : 12.0.7601.17514
    WMFSDKNeeded    : 0.0.0.0000
    IsVBR           : 0
    encoder         : Lavf54.22.100
    Stream #0:0: Video: h264 ([33][0][0][0] / 0x0021), yuv420p, 1280x720, q=-1--
1, 15 tbn, 15 tbc
Stream mapping:
  Stream #0:0 -> #0:0 (msmpeg4 -> libx264)

Conversion to MP4 with copy

ffmpeg -i input.wmv -c:v:1 copy output.mp4

  libavutil      51. 66.100 / 51. 66.100
  libavcodec     54. 49.100 / 54. 49.100
  libavformat    54. 22.100 / 54. 22.100
  libavdevice    54.  2.100 / 54.  2.100
  libavfilter     3.  5.102 /  3.  5.102
  libswscale      2.  1.100 /  2.  1.100
  libswresample   0. 15.100 /  0. 15.100
  libpostproc    52.  0.100 / 52.  0.100
Input #0, asf, from 'input.wmv':
  Metadata:
    WMFSDKVersion   : 12.0.7601.17514
    WMFSDKNeeded    : 0.0.0.0000
    IsVBR           : 0
    encoder         : Lavf54.22.100
  Duration: 01:14:23.06, start: 0.000000, bitrate: 324 kb/s
    Stream #0:0: Video: msmpeg4 (MP43 / 0x3334504D), yuv420p, 1280x720, 15 tbr,
1k tbn, 1k tbc
[libx264 @ 03437620] using cpu capabilities: MMX2 SSE2Fast SSSE3 FastShuffle Cac
he64
[libx264 @ 03437620] profile High, level 3.1
[libx264 @ 03437620] 264 - core 125 r2208 d9d2288 - H.264/MPEG-4 AVC codec - Cop
yleft 2003-2012 - http://www.videolan.org/x264.html - options: cabac=1 ref=3 deb
lock=1:0:0 analyse=0x3:0x113 me=hex subme=7 psy=1 psy_rd=1.00:0.00 mixed_ref=1 m
e_range=16 chroma_me=1 trellis=1 8x8dct=1 cqm=0 deadzone=21,11 fast_pskip=1 chro
ma_qp_offset=-2 threads=3 lookahead_threads=1 sliced_threads=0 nr=0 decimate=1 i
nterlaced=0 bluray_compat=0 constrained_intra=0 bframes=3 b_pyramid=2 b_adapt=1
b_bias=0 direct=1 weightb=1 open_gop=0 weightp=2 keyint=250 keyint_min=15 scenec
ut=40 intra_refresh=0 rc_lookahead=40 rc=crf mbtree=1 crf=23.0 qcomp=0.60 qpmin=
0 qpmax=69 qpstep=4 ip_ratio=1.40 aq=1:1.00
Output #0, mp4, to 'output.mp4':
  Metadata:
    WMFSDKVersion   : 12.0.7601.17514
    WMFSDKNeeded    : 0.0.0.0000
    IsVBR           : 0
    encoder         : Lavf54.22.100
    Stream #0:0: Video: h264 ([33][0][0][0] / 0x0021), yuv420p, 1280x720, q=-1--
1, 15 tbn, 15 tbc
Stream mapping:
  Stream #0:0 -> #0:0 (msmpeg4 -> libx264)

Conversion to AVI with copy

ffmpeg -i input.wmv -c:v:1 copy output.avi

Input #0, asf, from 'input.wmv':
  Metadata:
    WMFSDKVersion   : 12.0.7601.17514
    WMFSDKNeeded    : 0.0.0.0000
    IsVBR           : 0
    encoder         : Lavf54.22.100
  Duration: 01:14:23.06, start: 0.000000, bitrate: 324 kb/s
    Stream #0:0: Video: msmpeg4 (MP43 / 0x3334504D), yuv420p, 1280x720, 15 tbr,
1k tbn, 1k tbc
Output #0, avi, to 'output.avi':
  Metadata:
    WMFSDKVersion   : 12.0.7601.17514
    WMFSDKNeeded    : 0.0.0.0000
    IsVBR           : 0
    ISFT            : Lavf54.22.100
    Stream #0:0: Video: mpeg4 (FMP4 / 0x34504D46), yuv420p, 1280x720, q=2-31, 20
0 kb/s, 15 tbn, 15 tbc
Stream mapping:
  Stream #0:0 -> #0:0 (msmpeg4 -> mpeg4)

Are there some additional parameters I need to pass?

Best Answer

Stream copying

When you call -c:v:1 copy, FFmpeg will take the existing video bitstream and stream copy it. The video bitstream is just encapsulated in the outside container, e.g. WMV, AVI or MP4 – your actual video bitstream is msmpeg4 and will stay like this.

If you want to know more about what I'm talking about, see here: What is a Codec (e.g. DivX?), and how does it differ from a File Format (e.g. MPG)?

When copying the bitstream, FFmpeg doesn't need to actually decode and re-encode the actual video. It just needs to merge the video bitstream into a new container format, which is often a rather simple operation and therefore doesn't take long.

Encoding

In contrast to that, if you call -vcodec libx264 (or -c:v libx264, the syntax you should use because vcodec is deprecated), FFmpeg will be forced to decode the video bitstream from msmpeg4 to a raw format, then pipe it into x264, a H.264 encoder.

x264 is fast, but still, encoding video takes time – especially when it's 720p content. And it might take more than one hour, especially if your input is already longer than one hour. Also, your CPU might not be the fastest. This is the main reason older MPEG-4 Visual encoders like XviD are still around and very popular: They take less time to encode than H.264 codecs. They might not give you the best performance in terms of quality vs. file size, but they are fast.

That all being said: You can speed up x264 encoding by forcing a preset. Presets are encoder optimization settings and range from: ultrafast, superfast, veryfast, faster, fast, medium, slow, slower, veryslow. Your command could then look like this:

ffmpeg -i input.wmv -c:v libx264 -preset fast out.mp4

It should run faster than without the preset. The only drawback is that it doesn't achieve as good quality for the same compression rates in comparison to, for example, -preset veryslow.

Apart from that, there's not much you can do except for investing in a speedy CPU, and making sure you're running a recent build of FFmpeg with x264 support.

For more info see FFmpeg Wiki: H.264 Encoding Guide.